At the combustion of black liquor in the recovery boiler in a sulphate pulp plant there is obtained a molten residual product together with the sodium sulphate which is supplied to replace the consumption of chemicals. This consists substantially of sodium carbonate and sodium sulphide and is continuously discharged through a number of spouts, the so called smelt spouts, from the bottom portion of the recovery boiler. This stream of molten material is dissolved in an aqueous solution in a soda dissolving tank. The aqueous solution may consist of weak white liquor from the causticizing plant. The produced solution is pumped to the causticizing plant for further treatment.
The smelt has a temperature of about 900.degree. C. as it is discharged from the recovery boiler. It is therefore not possible to let this molten stream pour down directly into the aqueous solution in the dissolving tank, as this would cause explosions. It is therefore conventional to disintegrate the molten stream into small droplets before the smelt is allowed to come into contact with the aqueous solution. Further it is conventional to use water steam for the disintegration of the smelt from the recovery boiler. However, steam is a valuable product due to its energy content. The consumption of steam for smelt desintegration therefore is a considerable cost factor in the chemical recovery.
It is from other technical fields also known to desintegrate streams of molten material, for example molten metal, by means of a jet of water. This technique, however, is not useful for desintegrating the stream of molten smelt from the smelt spout because of explosion hazard involved when a water jet is brought into contact with the smelt.